In a wireless device (e.g., a keyless entry receiver) for home or vehicle use, radio waves having a relatively long wavelength from a few tens of centimeters to a few meters are used. For example, the wireless device generally uses ultrahigh frequency waves (UHF) or very high frequency waves (VHF). Therefore, size of the wireless device depends on size of an antenna used in the wireless device and the size of the antenna needs to be reduced to reduce the size of the wireless device.
A small antenna is disclosed in, for example, JP-A-2003-152427. The antenna includes a straight inner conductor and a helical outer conductor wound around the inner conductor with a space. The antenna resonates at a specified frequency. The antenna has a relatively high gain in spite of the fact that the antenna has a small and simple structure.
In the antenna, it is difficult to reduce the size of the antenna below a predetermined level. For example, when the antenna is reduced in length or width, the inner conductor or the outer conductor needs to be increased in height to obtain an electrical length for allowing the antenna to resonate. However, because the inner conductor is straight, the antenna is greatly increased in height if the inner conductor is increased in height.